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8/10/2019 Min Lecture 11.06.ppt
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Lecture 11 (10/18/2006)
CrystallographyPart 4:
Crystal Forms
Twinning
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Crystal Forms
Habit:the general externalshape of a crystal (e.g.,prismatic, cubic, bladed...)
Form: refers to a group offaces which have the same
relation to the elements ofsymmetry.
Crystals with higher degreesof symmetry tend togenerate more form faces.
openform
closedform
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Unique Attributes of Crystal Forms
NaClFaces of particular forms commonlyshare unique physical or chemicalattributes
Quartz
Anthophyllite
Pyrite
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Form Indexes
defined by the Miller
index (hkl) of the face
in the positive
quadrant enclosed in brackets
{hkl}
a{100},r {111},c {001}
Tetragonal 4/m 2/m 2/m
a1
a2
c
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Pyramids and Dipyramids
Prisms
Low Symmetry Forms
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Trapezohedron, Scalehedron,
Rhombehedron, Disphenoid
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Isometric
Forms
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Twinning
Symmetrical intergrowth of two or more crystalsrelated to a symmetry operation (twin element)that is otherwise absent in a single crystal.
Twin elements includes mirrors, rotation axes(usually 2-fold) and roto-inversion that usually donot align with symmetry elements in the crystal.
Twin Laws define the twin element and itscrystallographic orientation (twin planes areidentified by its Miller index (hkl), twin axes aredefined by a zone symbol [hkl]).
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Twin TypesPotential
Twin Plane (111)
Mirror
Reflection
Composition
PlaneContact TwinsCompositionplanescorrespond to twinplanes (mirrors)
Penetration TwinsCompositionplanes areirregular;formed by
rotational twinaxis operation
Twin
Axes
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Multiple TwinsFormed from 3or more twinnedcrystals
- Polysynthetictwins wherecomposition
planes areparallel
- Cyclic twinswherecomposition
planes are notparallel
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Twin Laws in the Triclinic System
Albite Law{010} twin plane
Albite combinedwith Pericline Law[010] twin axis
results in tartantwinning in microcline(K-feldspar)
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Twin Laws in the Monoclinic
System
Note that twin planes do
not align with mirror planes
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Twin Laws in the Orthorhombic Systemcontact & cyclictwinning
Contact or Penetrative??
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Twin Laws in the Tetragonal
System
T i L i th
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Twin Laws in the
Hexagonal System{0112} is most commonand may produce singlecontact or polysynthetictwins
exception:
twin axis || C
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Twin Laws in the Isometric System
Penetrativetwins withtwin axesparallel torotationalaxes
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Next Lecture
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